(1) Field of the Invention
This invention relates to a radiological image area extracting apparatus, a radiological image area extraction program and a radiographic apparatus for selecting an area of interest of an object under examination from radiological images picked up.
(2) Description of the Related Art
In order to secure images suitable for diagnosis, a radiographic apparatus extracts an area of interest of a patient from radiological images obtained by detecting radiation (e.g. X-rays or gamma rays), and carries out various image processes on the extracted area of interest of the patient. In radiographing the chest of the patient, what is needed particularly for diagnosis is a lung field area in the radiological images. In the radiological images, however, the abdomen and arms unnecessary for diagnosis and areas directly irradiated with radiation will also be included along with the lung field area required for diagnosis. These areas are greatly different in image density and contrast from the area of interest which is the lung field area of the patient. In order to carry out image processes with increased efficiency, it is required to exclude the areas other than the area of interest from areas subjected to the image processes (see Japanese patent No. 4143149, for example).
According to Japanese patent No. 4143149, in order to extract the lung field area of a patient first, a process is carried out to binarize an image based on a threshold, and a labeling process is carried out for the binarized image. Then, among a plurality of areas subjected to the labeling process, areas below a predetermined size are removed as areas showing high density outside the lung field area, and areas adjacent the top, bottom, left and right ends of the image are removed as areas directly irradiated with X-rays. On the other hand, areas not removed are selected as lung field area. A threshold is estimated from a maximum pixel value and a minimum pixel value, in which, for example, the maximum pixel (density) value is calculated from the image area of the patient from which the directly irradiated areas are removed, and the minimum pixel value is calculated from recesses of a profile passing through the coordinates of this maximum pixel value.
Another apparatus of this type is disclosed in Japanese Unexamined Patent Publication No. 2002-369079, for example. According to this publication, in order to remove a background space (directly irradiated area) providing no diagnostic information from a mammographic image first, a binary image is produced based on a threshold. Then, skin line pixels are determined to select an area.
Such a conventional apparatus sets a threshold for removing the directly irradiated area, and an area value for excluding areas below a predetermined area which have been extracted in error, or a threshold for selecting an area of interest of a patient. However, the conventional apparatus has a drawback that, when subjects of radiography or radiographing conditions are changed, for example, area extraction cannot be carried out with high accuracy unless optimal values are set all over again each time such a change takes place.
There is a further drawback that, under the influence of the threshold, noise and so on, the area selected can be altered and variations can occur in accuracy. Further, areas are detected by the binarization process and labeling process of an entire image, and these processes are time-consuming and susceptible to the influence of noise, for example.